GENERAL EMBRYOLOGY 1625 



The following account of the mode of formation of the amnion in 

 the chick, although referred to already, may here be re-stated. 

 The amnion is developed from the somatopleure which forms the 

 outer elevated lip of the marginal or circular groove round the 

 embryonic area of the blastodermic vesicle, the appearance of 

 which groove constitutes the first stage in the separation of the 

 germinal disc. This somatopleure, which is disposed in two layers, 

 inner and outer, gives rise to the amniotic folds. The circular 

 groove deepens and encroaches, from all sides, upon the ventral 

 aspect of the embryonic area as far as the site of the future umbilicus ; 

 and the somatopleure of the inner wall of the groove forms the 

 lateral and ventral body- walls of the embryo. Meanwhile the 

 amniotic folds gradually encroach, from all sides, upon the dorsal 

 aspect of the embryonic area in the form of cephalic, lateral, and 

 caudal hoods. Subsequently these hoods meet and fuse. When 

 this fusion has taken place, the inner layer of somatopleure separates 

 from the outer layer (except at the caudal or posterior extremity 

 of the embryo), and it constitutes a closed sac, which is the amnion, 

 the outer layer of somatopleure giving rise to the chorion. 



The wall of the amnion is formed by somatopleure — that is to say, 

 ectoderm and somatic mesoderm. The ectodermic layer is the 

 inner of the two, and forms the lining of the amniotic cavity, whilst 

 the layer of somatic mesoderm is situated externally. The body- 

 wall of the embryo, as stated, also consists of somatopleure ; but in 

 this case the ectoderm is external, and the somatic mesoderm lies 

 on its inner aspect. The ectodermic layer of the wall of tl.e amnion, 

 which is the inner layer, therefore faces the ectodermic laver of the 

 body-wall of the embryo, which layer is situated on the exterior. 

 At the site of the future tmibilicus, on the ventral aspect of the 

 embryo, the somatopleure forming the wall of the amnion becomes 

 continuous with the somatopleure which constitutes the body-wall 

 of the embryo. 



In the human embryo the amnion is formed in a manner different 

 from that which has just been described as applicable to the chick. 

 In the chick the amnion, briefly stated, is formed from the amniotic 

 folds, representing the outer elevated lips of the circular groove 

 around the embryonic area. The four amniotic folds — namely, 

 cephalic, two lateral, and caudal — come together over the embryonic 

 area, and by their fusion give rise to the amnion, which is therefore 

 not a closed sac at first. In the human embr\-o it is different. 

 From the very commencement the amnion is a closed sac, and is 

 formed within the inner ceU-mass of the blastodermic vesicle. 

 Vacuoles form within the uppermost of these cells, the remainder 

 of the cells forming the embryonic bud or disc. Bv the coalescence 

 of these vacuoles the amniotic cavity is formed, "which is thus a 

 closed cavity from the first. The cells surroimding this ca\'ity, and 

 therefore forming its lining, are derived from the embrvonic ecto- 

 derm, which forms the inner layer of the wall of the amnion. The 

 outer layer of the waU, formed by somatic mesoderm, is derived 



